Electronic surveillance measurement (ESM) receivers commonly require that frequency calculations be performed on a received signal from targets of interest. The frequency of an input signal is often measured using an instantaneous frequency measurement (IFM) receiver. The IFM receiver, generally, uses a difference in phase between a delayed and a non-delayed version of the input signal to calculate the frequency of the input signal.
Two conventional IFM receivers are shown in FIGS. 1a and 1b. Referring first to FIG. 1a, a signal received from an antenna and/or an amplifier (not shown) is input into power divider (PD) 11a. A first signal output from the power divider is passed through an analog delay line, generally designated as 12a, to form a delayed signal characterized by cos(ωt−ωτ). The delayed signal is then passed through a 90° hybrid, generally designated as 13a, to form cos(ωt−ωτ) and sin(ωt−ωτ). A second signal output from power divider 11a, characterized by cos ωt is passed through a 180° hybrid, generally designated as 15a, to form a signal of cos ωt and another signal of −cos ωt.
As also shown in FIG. 1a, system 10 passes the aforementioned signals through two additional 90° hybrids, generally designated as 13b and 13c, and onto four video detectors, generally designated as 14a, 14b, 14c and 14d. Video output signals, A2 and B2, are digitized and correlated by phase correlator 16a. Similarly, video output signals, C2 and D2, are digitized and correlated by phase correlator 16b. The correlated output signals, at points E and F, are provided to a processor (not shown) for further processing to determine the frequency of the input signal into system 10.
Another IFM receiver, generally designated as 17, is shown in FIG. 1b. As shown, the input signal is passed through power divider 11b to form two signals. One of the signals is delayed by an analog delay line, generally designated as 12b, and further divided by power divider 11c. The other signal, output from power divider 11b, is further divided by power divider 11d. These four divided output signals are, respectively, passed through a 90° hybrid, generally designated as 13d, and a 180° hybrid, generally designated as 15b. The output signals from the hybrids are then video detected by video detectors 14e, 14f, 14g and 14h. The four video output signals are then digitized and phase correlated by phase correlators 16c and 16d. As described with respect to FIG. 1a, the correlated output signals, at points E and F are further processed by a processor (not shown) to determine the frequency of the input signal received by system 17.
Conventional IFM receivers use analog components, such as hybrids, power dividers and crystal video detectors, as illustrated in FIGS. 1a and 1b, to convert a received input signal into video signals. These video signals are further processed to find the frequency of the received input signal. The frequency is obtained through phase measurement of the input signal and its delayed version. The basic functional building blocks of a conventional IFM receiver includes a correlator, as provided in system 10 and system 17.
System 10 and system 17 provide different approaches to determining the input frequency of a received signal. In system 10, three 90° hybrids and one 180° hybrid are used. In system 17, on the other hand, two 90° hybrids are shown replaced by two power dividers. In both system 10 and system 17, the input signal is divided into two paths, one path is delayed by a known delay time τ, through an analog delay line, as shown. In both systems, the video signals are digitized and correlated. In building an IFM receiver, multiple correlators with different delay line lengths are typically needed. For example, some receivers may use four correlators and other receivers may use up to seven correlators.
It would be advantageous, if the number of delay lines required could be reduced. It would also be advantageous if the number of hybrids could be reduced. Furthermore, it would be advantageous if the number of crystal video detectors could be reduced.
The present invention provides such advantages by having a reduced number of components. As will be explained, analog delay lines are not necessary for the present invention. As will also be explained, the present invention only requires one 90° hybrid, and does not require any crystal video detectors.